This invention is directed to a power transmission arrangement and in particular to the provision of a multipowered heavy duty drive system incorporating hydraulic clutches and associated controls particularly adapted for use in conjunction with large grinding mills.
As the capacity of large scale grinding mills such as ball, rod or autogenous mills is increased, it becomes increasingly difficult to provide an efficient trouble free drive system therefor of sufficient size. Accordingly, it has become common to employ twin drive systems operative to drive a single mill. Such drive systems incorporate higher power electric motors such as synchronous motor for example and have utilized air actuated clutches in order to permit synchronization of the drive motors. Such a drive system is disclosed in Nelson, U.S. Pat. No. 3,369,636.
While such air actuated clutches are suited for use in rapidly accelerating the mill up to and maintaining operation at full speed, their limited heat dissipation capability severely limits the frequency, duration, and extent of clutch actuation.
In the operation of such mills certain novel phases of activity could be achieved by provision of a greatly enhanced clutching capability over that presently afforded by air clutches. Included in these desired activities is the angular repositioning of the mill drum by inching to permit servicing or replacement of the mill liners.
The provision of a hydraulic clutch or clutches of the wet plate type referred to as wet clutch, having an external coolant circuit or circuits introduce certain unobvious advantages over the previously known use of air clutches and provide the flexibility to incorporate substantially increased flexibility into the operational modes of the grinding mill control systems.
Thus, the wet clutch may be utilized in a distinctly different manner by incorporating in its control program for normal running a closely toleranced overload setting, at which the clutch will slip, to thus limit the gross value of torque which may be transmitted. An alarm or an automatic shutdown or automatic mill off loading arrangement also can be incorporated with this control mode. Thus, once clutch lock up occurs the torque transmission capability could be increased to a suitable preselected value above normal full load torque but below the maximum torque transmission capability to the drive systems components. This limit of maximum torque load affords protection of the reduction gear or gears and other drive system components against damage due to overload. One previous solution to this problem of over-torquing relied upon shear pins installed in combination with a solid coupling. However, this prior arrangement suffered from the disadvantage that shear pin failure could occur due to fatigue, as distinct from torque overload, without approaching the pre-set value for overload. Thus, failure of a shear pin by fatigue, as distinct from torque overload, could cause the expense of a mill shutdown and require the expenditure of time and labor for refitting of a new shear pin or pins. This is unduly time consuming and costly in light of the production loss suffered by mill shutdown.
A further novel function for the system, made possible by the heat dissipation characteristics of the wet clutch, is the provision of an inching drive wherein the driving motor or motors may be connected in driving relation to the mill through the clutches under a slip condition for a predetermined limited period of time, in order to achieve a limited predetermined degree of mill rotation. The period of clutch activation and the degree of slip may be selectively varied.
Alternatively, pulsed inching can be achieved by cyclically engaging and disengaging the clutch so as to provide a slow, pulsed forward motion.
This is of particular value in carrying out mill maintenance, such as servicing or changing mill liners, wherein it is sometimes necessary for the maintenance crew to replace liners within the mill while standing upon the charge. The capability of precise inching to a predetermined degree of rotation affords a significant economic advantage in reducing mill downtime required for such periodic servicing.
A further desirable capability for such mills is the ability to rapidly stop and lock the mill drum against rotational movement such as for servicing, recharging or the like. Controlled energization of the clutch or clutches can be employed to effect a braking action to thereby minimize the duration of pendulum-like oscillating motion often encountered during mill shutdown. This can also significantly speed up the repositioning of the drum during internal maintenance and service work, and enhance safety.
It is further contemplated, that while continuous clutch slip is generally to be avoided on the grounds of obtaining efficient energy utilization, in certain circumstances where the mill charge received from the mine varies outside generally accepted tolerances so as to adversely affect mill operation, short term corrective action can be taken by permitting a selected degree of clutch slip while maintaining operation of the driving motors at normal operating speed.
By making provision for clutch cooling to a greater extent than is normally considered necessary, mill rotation at 90% or even 80% of the normal operational speed is made possible, while maintaining full speed operation of the motor.
The present invention thus provides a drive arrangement for use with a grinding mill having a drum mounted for rotation on bearing means, a driven gear secured to the drum, at least one pinion supported in driving relation with the driven gear, and electric motor means comprising wet clutch means interposed in selective connecting relation between the motor means and the pinion, the wet clutch means including a coolant circuit to dissipate heat generated in the clutch means, and mill control means including a clutch control circuit.
The invention further provides inching means to control the actuation of the wet clutch for a controlled period of time, to provide a selectively variable or a predetermined extent of rotation of the drum by the electric motor.
The invention also includes the provision of braking means to immobilize the output half of the clutch whereby subsequent to actuation of the wet clutch a braking torque is applied to the mill drum to bring it to a standstill.
The foregoing provisions of load monitoring, load balancing and load limiting may be effected utilizing a suitable electrical and electrohydraulic load monitoring and control circuits which preferably will be of solid state construction and may employ a micro-process computer to selectively control operation of the clutch. The controller circuits can be designed to provide an automatic controlled rate of mill acceleration, from standstill, by monitoring motor load and modulating the respective clutch load control pressures accordingly both to control the loading rate and to maintain load balance of multiple motors of a multiple drive system within predetermined limits.
In accordance with one embodiment of the invention, upon the occurrence of load imbalance between the two motors, beyond a predetermined limit during normal running, the clutches are depressurized to a sufficient extent that continuous slippage occurs whereat the speed of mill rotation may decrease by a value such as 5% of operating speed. With the mill thus operating at about 95% full speed for instance, one of the clutches is modulated so as to equalize the loading of the motors and an acceleration schedule then initiated to bring the mill back up to full speed.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.